In general, in a process for producing a semiconductor device, a micro pattern is formed by photolithography. Furthermore, the forming of this micro pattern usually uses several substrates called photomasks. In general, these photomasks are each made of a light-shielding metal thin film or the like having a micro pattern on a light-transmitting glass substrate, and photolithography is also used in the production of these photomasks.
The production of a photomask by photolithography uses a photomask blank having a light-shielding film on a light-transmitting substrate such as a glass substrate. This method of forming a photomask by using a photomask blank includes an exposure process of subjecting a resist film formed on the photomask blank to predetermined patterning exposure, a development process of forming a resist pattern by developing the resist film according to the predetermined patterning exposure, an etching process of etching the light-shielding film along the resist pattern, and a process of removing the remaining resist pattern. In the development process, the resist pattern is formed by subjecting the resist film formed on the photomask blank to predetermined patterning exposure and then supplying a developing solution for dissolving the resist film portion, which is soluble to the developing solution. In addition, in the etching process, a desired mask pattern is formed on the light-transmitting substrate by using the resist pattern as a mask, and dissolving the exposing portion of the light-shielding film where the resist pattern is not formed by, for example, wet etching. Thus, a photomask is formed.
Patent Document 1 describes a photomask blank as a mask blank suitable for wet etching. The photomask blank includes a transparent substrate having a light-shielding film of a chromium film containing chromium carbide thereon. Similarly, Patent Document 2 describes a half-tone phase-shift mask blank as a mask blank suitable for wet etching. The half-tone phase-shift mask blank includes a lamination film of a half-tone material layer and a metal layer on a transparent substrate. This metal layer includes a region constituted with different materials so that the etching grade varies from the surface side toward the transparent substrate side. For example, a lamination film composed of a metal layer of CrN/CrC and a reflection-preventing layer of CrON is described.
In a case of a chromium light-shielding film as described in Patent Documents 1 and 2, it is known that crystalline granules pull to each other when the film is formed, and thereby a film stress occurs in the tensile direction. It is also known that the problem of occurrence of a film stress is significant particularly in a case of a chromium carbide film containing carbon in chromium or a chromium oxide film containing oxygen in chromium. In a photomask blank including a light-shielding film thus having a film stress, a warp occurs in the substrate. If a photomask is formed using this photomask blank having low flatness due to this warp of the substrate, a designed patterning accuracy cannot be achieved. If the pattern is transferred onto a semiconductor substrate (silicon wafer) using such a photomask, a designed pattern is not precisely formed on the semiconductor substrate to cause bad operation. This problem has been aggravated with recent miniaturization of a pattern.
Therefore, a chromium light-shielding film having a film stress of sufficiently near zero (0) is ideal. For example, Patent Document 3 proposes a reduction in the film stress of a chromium light-shielding film.
That is, Patent Document 3 describes a method of producing a photomask blank provided with a light-shielding film having a low film stress by introducing helium into an atmospheric gas during the formation of the film.    Patent Document 1: Japanese Patent Publication (JP-B) No. 62-32782    Patent Document 2: Japanese Patent (JP-B) No. 2983020    Patent Document 3: Japanese Patent (JP-B) No. 3276954